"Last month, Facebook Security discovered that our systems had been targeted in a sophisticated attack," they wrote.

"This attack occurred when a handful of employees visited a mobile developer website that was compromised. The compromised website hosted an exploit which then allowed malware to be installed on these employee laptops. The laptops were fully-patched and running up-to-date anti-virus software. As soon as we discovered the presence of the malware, we remediated all infected machines, informed law enforcement, and began a significant investigation that continues to this day."

The key sentence for Facebook users: "We have found no evidence that Facebook user data was compromised."